When converting ns,us,ms to sbt, return the ceil()

of the result rather than the floor(). Returning the floor means that
sbttoX(Xtosbt(y)) != y for almost all values of y. In practice, this
results in a difference of at most 1 in the lsb of the sbintime_t. This
difference is meaningless for all current users of these functions, but
is important for the newly introduced sysctl conversion routines which
implicitly rely on the transformation being idempotent.

Sponsored by: Netflix, Inc

1 files changed, 7 insertions, 3 deletions

diff --git a/newlib/libc/include/sys/time.h b/newlib/libc/include/sys/time.h
index e5023ae64..5182dc14e 100644
--- a/newlib/libc/include/sys/time.h
+++ b/newlib/libc/include/sys/time.h
@@ -171,6 +171,10 @@ sbttobt(sbintime_t _sbt)
  * Decimal<->sbt conversions.  Multiplying or dividing by SBT_1NS results in
  * large roundoff errors which sbttons() and nstosbt() avoid.  Millisecond and
  * microsecond functions are also provided for completeness.
+ *
+ * These functions return the smallest sbt larger or equal to the number of
+ * seconds requested so that sbttoX(Xtosbt(y)) == y. The 1 << 32 - 1 term added
+ * transforms the >> 32 from floor() to ceil().
  */
 static __inline int64_t
 sbttons(sbintime_t _sbt)
@@ -183,7 +187,7 @@ static __inline sbintime_t
 nstosbt(int64_t _ns)
 {
 
-	return ((_ns * (((uint64_t)1 << 63) / 500000000)) >> 32);
+	return ((_ns * (((uint64_t)1 << 63) / 500000000) + (1ull << 32) - 1) >> 32);
 }
 
 static __inline int64_t
@@ -197,7 +201,7 @@ static __inline sbintime_t
 ustosbt(int64_t _us)
 {
 
-	return ((_us * (((uint64_t)1 << 63) / 500000)) >> 32);
+	return ((_us * (((uint64_t)1 << 63) / 500000) + (1ull << 32) - 1) >> 32);
 }
 
 static __inline int64_t
@@ -211,7 +215,7 @@ static __inline sbintime_t
 mstosbt(int64_t _ms)
 {
 
-	return ((_ms * (((uint64_t)1 << 63) / 500)) >> 32);
+	return ((_ms * (((uint64_t)1 << 63) / 500) + (1ull << 32) - 1) >> 32);
 }
 
 /*-